Plasmonic platforms for innovative surface plasmon resonance configuration with sensing applications

Microelectronic Engineering 111 (2013) 348–353

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Plasmonic platforms for innovative surface plasmon resonance configuration with sensing applications E. Pasqualotto a,⇑, G. Ruffato b,c,d,⇑, A. Sonato b,c, G. Zacco b,c,d, D. Silvestri e, M. Morpurgo e, A. De Toni a, F. Romanato b,c,d a

University of Padova, Department of Information Engineering, via Gradenigo 6, 35131 Padova, Italy University of Padova, Department of Physics and Astronomy, via Marzolo 8, 35131 Padova, Italy Laboratory for Nanofabrication of Nanodevices, LaNN – Venetonanotech, CorsoStatiUniti 4, 35127 Padova, Italy d CNR-INFM TASC IOM National Laboratory, Area Science Park, S.S. 14 km 163.5, 34012 Basovizza, Trieste, Italy e University of Padova, Department of Pharmaceutical Sciences, Via Marzolo 5, 35131 Padova, Italy b c

a r t i c l e

i n f o

Article history: Available online 14 March 2013

a b s t r a c t An experimental prototype exploiting Grating-Coupled Surface Plasmon Resonance (GCSPR) based on polarization modulation has been assembled and tested for sensing purposes. The plasmonic gratings are azimuthally rotated in order to exploit the symmetry breaking for the excitation of highly sensitive Surface Plasmon Polaritons in conical mounting. By exploiting the optimal-polarization shift, a scan of the incident polarization is performed and reflectivity data are collected. The output signal exhibits a harmonic dependence on polarization and the phase term is considered as a parameter for sensing. Since the optical configuration is fixed during the analysis and the only degree of freedom is represented by the incident polarization, this setup provides a more compact and simplified architecture with respect to other commercial SPR techniques, however assuring at the same time competitive performances in refractive index sensitivity and resolution. The employed metallic gratings are fabricated by interferential lithography and replicated onto resin substrates by soft-lithography techniques, thus thermally evaporated. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Nowadays new detection technologies aim to miniaturize the sensing devices and, at the same time, to increase sensitivity and resolution [1]. Several studies focus on the realization of a labon-chip system that allows processing the sample and detecting very low concentrations of target molecules, which means not only an excellent sensitivity performance but also a reduction in terms of time processing and of reagent costs. Thus research moves towards the direction of label-free sensing platforms which should be particularly suitable to miniaturization and embodiment into a microfluidic analysis system. Surface plasmon polaritons are considered to be an excellent resource for surface analysis, thanks to their great sensitivity due to the localized nature and the presence of suitable supporting platforms. Surface Plasmon Polaritons (SPPs) are evanescent modes propagating along the interface between a metal and a dielectric medium and have origin in the coupling between electromagnetic waves and electron-plasma oscillations inside the metal [2]. SPP excitation has the important feature to ⇑ Corresponding authors. Address: University of Padova, Department of Physics and Astronomy, via Marzolo 8, 35131 Padova, Italy (G. Ruffato). E-mail addresses: [email protected] (E. Pasqualotto), gianluca.ruffato@ unipd.it, [email protected] (G. Ruffato). 0167-9317/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2013.02.088

be extremely sensitive to surface changes and a functionalization of the metal surface can be detected through variations of the plasmonic resonance. This is the basic principle of modern Surface Plasmon Resonance (SPR) sensing devices [3] that in the last decades have been used in several field: environmental protection, biotechnology, medical diagnostics, drug screening, food safety and security [4]. Common SPR affinity biosensors is composed of a biorecognition element, immobilized over the metallic sensor surface, which selectively interacts with the target analyte, and a SPR transducer, which translates the refractive index variation, due to the binding event, into an output signal. The transducer consists of an optical platform on which surface plasmon polaritons are optically excited and propagate. There exist different configurations in order to excite the SPPs and detect refractive index variations. For example, sensors based on Prism-Coupled SPR (PCSPR) with Kretschmann configuration [5] can be readily combined with any type of interrogation: angular, wavelength, intensity or phase modulation [6], but they are not suitable for miniaturization and integration. For this purpose, a good alternative is given by Grating-Coupled SPR (GCSPR) sensors with either wavelength or angular interrogation. Several studies have demonstrated GPSPR sensors to have a sensitivity 2–3 times lower than PCSPR [7], however they have the possibility to be used

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with different sensing architectures and interrogation systems that can further improve their sensing performance. Homola’s group demonstrated a miniaturized GCSPR sensor implemented with a CCD allowed detection sensitivity of 50°/RIU and resolution of 5  106 over 200 sensing parallel channels [8]. Alleyne has exploited the generation of an optical band gap by using prismcoupled to achieve sensitivity down to 680°/RIU by bandgap-assisted GCSPR [9]. A recent approach was reported by Telezhnikova and Homola [10] with the development of a sensor based on spectroscopy of SPPs down to 5  107 RIU. This work focuses on the development of an innovative gratingcoupled plasmonic sensor with competitive performance in terms of sensitivity and resolution and a compact interrogation system. In our recent works we studied the effects of symmetry breaking by azimuthal rotation on surface plasmon excitation and propagation and the results were an increment of excited SPPs at the same wavelength [11] and a sensitivity up to 1000°/RIU for the second dip in angular interrogation [12], which is one order of magnitude greater than that in the non-rotated conventional configuration. In this type of interrogation system, SPPs are excited not only by p-polarization but also by s-polarization, so the incident polarization acquires a central role on SPP excitation [13]. A further result is that the optimal polarization strictly depends on the resonance conditions and it can be considered as a parameter for sensing analysis [14]. Therefore, in this work, a custom sensing setup has been developed where surface plasmon resonance is based on polarization modulation instead of angular or wavelength interrogation. The polar and azimuthal angle can also be varied in the setup, but they are kept fixed during the analysis, reducing the complexity of the measurement in comparison to conventional PCSPR or GCSPR devices, however assuring a competitive performance in sensitivity and resolution in accordance to commercial SPR systems. In this work the polarization-based sensing configuration has been first tested for the detection of a bulk refractive index variation in water solution flowing through a microfluidic cell and then it has been exploited to detect a specific analyte in solution, i.e. avidin molecules. This detection has been achieved thanks to the immobilization of a first antifouling layer of Poly(ethylene oxide) (PEO) polymer chain [15,16], bound to the gold plasmonic platform via a Cysteine-terminated group, and with the other functional end linked to a biotin molecule, which is known to assure a great affinity and specificity to avidin in biological solutions [17].

2. Materials and methods 2.1. Gratings nanofabrication Nanofabrication of sinusoidal gratings was carried out through substrate lithography (by Laser Interference Lithography – LIL), nanostructure replica and thermal evaporation of the plasmonic layer [18,19]. A bottom anti-reflection coating (XHRiC-11, BARC) was spun on a pre-cleaned silicon surface at 3000 rpm for 30 s. After a soft baking step (175 °C for 1’), the substrate was coated with a S1805 film obtained from a S1805 (MicropositÒ, Shipley European Limited, U.K.): PGMEA (propylene glycol monomethyl ether acetate) (2:3) solution, with a spin speed of 3000 rpm for 30 s. The exposure step was performed setting the laser incidence angle at 19° and the exposure dose at 40 mJ/cm2. Exposed regions were removed by immersing the sample in Microposit MF-321 developer for 30 s. In a further step, a polydimethylsiloxane (PDMS-Sylgard 184 10:1) mold, obtained from the LIL master, was employed to imprint the nano-pattern onto thiolene resin (Norland Optical Adhesive-NOA 61) replica, supported onto microscope glass slides. Finally an optimized metal bi-layer of chromium

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(5 nm)/gold (40 nm) was evaporated above the resin replica (Fig. 1a and b). 2.2. Microfluidic cell fabrication A microfluidic cell embodied into the plasmonic platform was fabricated by soft-lithography technique using commercial thiolene resin optical adhesives [20] (Fig. 2). The resulting structure is a two-level resin cell on a glass substrate, consisting of a gasket with an aperture in the detection zone and inlet ports distant from the detection region so that tube connectors can be placed farther apart and do not obstruct the optical path. The first bottom layer is fabricated as it follows (see Fig. 2 for a schematic description): thiolene monomer liquid NOA61 (Norland Products Inc. Cranbury USA) is inserted between two surfaces, being the upper surface a thin, transparent polyethylene (PE) sheet, while the lower surface a microscope glass slide. The two surfaces are kept separated by spacers. The monomer liquid is photopolymerized with UV light (365 nm, 5000-PC DYMAX UV-light lamp) with a transparency mask to locally solidify the adhesive in the exposed zones [21]. The UV exposure dose is chosen high enough to solidify the whole layer thickness but not so high to produce full curing of the adhesive to the PE sheet (‘‘partial-curing’’). The uncured residual liquid is then removed by rinsing with a solution 1:1 of acetone and ethanol followed by drying under a nitrogen stream. The same procedure can be repeated in order to reproduce any other thiolene pattern that in turn is bound to the multilayered cell. Thus the second and top layer is fabricated by repeating the above steps with a thin layer of NOA enclosed between two PE sheets. After exposure, the supporting PE sheets can be easily peeled away, leaving the thiolene pattern to be applied on the substrate. To improve the adhesion of the two layers, the whole setup is finally exposed to UV radiation (‘‘hard-curing’’). 2.3. Spectroscopic ellipsometry Analyses were performed with a spectroscopic ellipsometer VASE (Woollam), working range 270–2400 nm, 0.3 nm of spectroscopic resolution and angular resolution 0.001°. The goniometercontrolled optical bench was set at three different angles of incidence on the sample (50°, 60°, 70°) and ellipsometric angles w and D were recorded in the rotating-polarizer analysis setup (Rotating Analyzer Ellipsometry – RAE). Data were analyzed with W-VASE software (Woollam) considering a Cauchy model with Urbach tail adsorption term for the functionalization layers [22]. Reflectivity measurements of the grating samples in water environment were performed in the range 15–45°, step size 0.2° at the incident wavelength k = 800 nm. 2.4. Assembled prototype An experimental setup exploiting Grating-Coupled Surface Plasmon Resonance technique based on polarization modulation was assembled and tested (Fig. 1d and e). The system is designed in order to allow an azimuthal control of the grating orientation by means of a rotating goniometer embodied into a sample holder in horizontal position. The laser source with incident wavelength k = 635 nm (Edmund VHK, beam diameter of 1.1 mm, 4.9 mW of output power) is mounted on an optical bench (Thorlabs: SL20/ M, C1525/M, P14) with a fine control of the incident angle with respect to the direction normal to the sample holder (polar angle h). The grating holder consists on a two translation stages (Edmund: 56-794, Thorlabs: DT25/M) able to move the surface along the x and y directions, a small goniometer (Thorlabs: GN18/M) to finely adjust the incidence angle (±5°, accuracy 0.15°) and a metric rotation stage (Edmund: NT55-028) to change

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Fig. 1. SEM (a) and AFM (b) analysis of a typical sinusoidal gold grating: period 505 nm, amplitude 46 nm, rms roughness 1.5 nm. Gold layer (40 nm) over a chromium adhesion layer (5 nm). Scheme (c) and details (d and e) of the experimental setup: (1) fixed-wavelength laser (k = 635 nm), (2) polarizer, (3) rotating half-wave plate, (4) incidence angle h control, (5) translating sample holder with (6) azimuthal u control, (7) microfluidic cell and (8) photodiode array (or CCD) for reflectivity collection connected to an electronic chain for data transduction. During the polarization-modulation analysis, polar and azimuthal angles (h, u) are kept fixed in correspondence of the plasmonic resonance and a polarization scan of the angle a is performed.

the azimuth angle. Reflected light modulated by gratings is detected through commercial photodiodes array (Hamamatsu S4111, 16 elements) which converts the light into current. The array is mounted in a y–z axis translational stage (Edmund: E55025) that linearly moves it along the two axis, in order to align the laser beam and the active area of a photodiode. Photodetectors are individually addressable by a switch/control unit (HP 3488A), which is connected to a semiconductor parameter analyzer (HP 4156). Both the instruments are controlled by custom Labview software, which also provides data acquisition and storage. Once the polar angle has been fixed, a scan of the azimuthal angle should be performed in order to localize the resonance value for SPP excitation, which can be detected as a current dip. Afterwards the grating is rotated and kept fixed at the resonance position. A half-wave plate (Thorlabs: WPH10M-633) mounted on a motorized rotation stage (Thorlabs: PRM1/MZ8E) between source and sample-holder allows controlling the incident polarization and performing a scan in the selected range of angles. 2.5. Avidin/biotin reaction essay preparation

Fig. 2. Diagram of the soft-lithography process for the two-level cell fabrication.

The microfluidic cell was placed over the sample which was pre-cleaned in a basic peroxide solution (5:1:1 double distilled H2O, 30% H2O2 and 25% NH4OH) for 10 min, rinsed in double

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distilled water and dried under N2 flux. The whole has been held in place on the sample holder. Thiol-protected end-functionalized biotin-PEO-Cys (bPEO) was synthesized by coupling S-trityl-cysteine to the hydroxyl end of biotin-PEO Mw 5 KDa. The trityl group (trt) acts as protector from oxidation of the reactive cysteine thiol residue and it was removed immediately before deposition by dissolving the a-methoxy-x-trtcys-poly(ethyleneoxide) powder in the minimum amount of TFA (about 20 ll) for 20 min at room temperature. N2-saturated double distilled H2O was then added up to reach 1 mM final thiol concentration. The insoluble trt residue was removed by centrifugation (10,000g, 4 °C, 10 min). mPEO-Cys deposition was then carried out by flowing of the supernatant solution for 48 h. The biorecognition event was performed by injecting into the cell a 4 lg/ml avidin/dilution buffer solutions for 2 h over the bPEO-Cys-coated surface. The same procedures of b-PEO-Cys grafting and subsequent avidin binding were performed over a flat gold sample, fabricated by thermal evaporation of about 40 nm of gold on a chromium adhesion layer (about 5 nm) over a glass substrate (sodalime). Spectroscopic ellipsometry between 300 and 1200 nm (5 nm step) was recorded with VASE Spectroscopic Ellipsometer (Woollam) in order to get an estimation of the avidin layer thickness. 3. Theory At a given azimuthal rotation u of the grating supporting SPP excitation, two distinct dips in reflectivity may appear in correspondence of the resonance polar angles h± given by [11]:

0

k h ¼ arcsin @ cos u  K

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1  2 n2 eM k A  sin u n2 þ eM K

ð1Þ

where k is the illuminating wavelength, K is the grating period, eM is the permittivity of the metallic layer and n is the refractive index of the facing dielectric. After rearranging Eq. (1) we get the following expression for u as a function of the polar angle h:

! 2 A2  S2 þ sin h u ¼ arccos 2A sin h

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ures, there is a shift Da0 in the phase term: Da0 = (@ a0/@n)Dn. This result opens the route to a new GCSPR-configuration based on polarization interrogation in the conical mounting [14].

4. Results and discussion At first, the mechanism of grating-coupled SPR with polarization modulation was tested for the detection of a bulk refractive index variation in water solution flowing through an integrated microfluidic cell. A mass of 1.236 g of sodium-chloride was dissolved into 123.888 g of double distilled water, giving a corresponding refractive index variation [23] Dn = 1.914  103 RIU with respect to pure water. Since for the considered grating period K  500 nm, surface plasmon polaritons are not supported by the gold-water interface at the incident wavelength k = 635 nm, this test was performed on the ellipsometer stage for incident wavelength k = 800 nm. The sample holder was azimuthally rotated at u = 58.0° and a reflectivity analysis was performed in polar angle h scan before and after salt dissolution for incident p-polarization (Fig. 3, inset graph). The flowing of the saline solution causes the resonance angle to shift from 35.82° to 34.81°, resulting in a variation Dh = 1.01° ± 0.06°. The corresponding refractive index sensitivity is Sn,h = Dh/Dn = 527.7°/RIU with a resolution rn,h = rh/Sn,h around 1  104 RIU. If a polarization scan is performed in correspondence of the SPP resonance angle hres = 35.82°, a harmonic dependence of the reflectivity minimum is given, according to the model in Section 3. As Fig. 3 shows, the fitting curve strictly depends on the resonance conditions, i.e. on the refractive index of the solution flowing through the cell, and this results in a phase shift Da0 = 0.451° ± 0.004° after sodium-chloride dissolution. A deeper analysis of the sensing technique was considered by performing the polarization scan for different points of the reflectivity dip in the range 30–40°, step 0.2°, both for pure water and saline solution flowing through the cell. Fitting values are collected and shown in Fig. 4. Clear evidence of the fitting parameters dependence on the polar angle h is given. In particular, the phase shift Da0 increases for increasing polar angle up to 2°, which is around 5 times the value at resonance (hres = 35.8°). However it is recommended

ð2Þ

where A = k/K, S2 ¼ n2  eM =ðn2 þ eM Þ. At resonance, the minimum of reflectivity Rmin exhibits a harmonic dependence on the incident polarization a with a periodicity of 180°:

Rmin ¼ f0  f1 cos ð2a þ a0 Þ

ð3Þ

where f0, f1 and a0 are fitting parameters that depend on the incidence angles, the incident wavelength and on the optical properties of the stack (thickness and dielectric permittivity of each layer). By assuming that only the electric field component lying on the grating symmetry plane is effective for SPP excitation, we obtained an analytical expression [13] for the optimal polarization aopt as a function of the resonance azimuth angle u and polar angle h (p-polarization: a = 0°):

tan aopt ¼ tan u cos h

ð4Þ

In case of optimal polarization impinging on the grating surface, the coupling strength is maximized and the reflectivity depth is minimized, thus in Eq. (3) cos(2a + a0) = 1 and we get the relation a0 = 2amin + 2kp, being k an integer number. If the grating surface is functionalized, the effective refractive index neff of the dielectric medium changes and the resonance condition is different, thus, for fixed incidence polar angle h, there is a shift in the resonance azimuth ures. As a consequence of the change in the resonance angle

Fig. 3. Fixed polar angle h = 35.8°, azimuth u = 58.0°: polarization scan in the range 0–180°, step size 10°, for double-distilled water flowing through the microfluidic cell (blue solid line) and a saline solution (1.236 g of NaCl in 123.888 g of water – Dn = 1. 914  103 RIU – red solid line). Fit curve with Eq. (3). In the inset graph: reflectivity for polar angle scan in the range 28–43°, step 0.2°. Data collected with the ellipsometer VASE system, azimuth u = 58.0°, k = 800 nm, p-polarization. (For interpretation of color in Fig. 3, the reader is referred to the web version of this article.)

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Fig. 4. Fit parameters f0, f1 (a) and a0 (b) before (blue line) and after (red line) NaCl dissolution (Dn = 1.914e-3) in pure water, for different incidence polar angles in the range 30–40°, step size 0.2°. In fig. (b) variation Da0 of the phase term (green points), error bars multiplied by a factor 8. (For interpretation of color in Fig. 4, the reader is referred to the web version of this article.)

not to fix the polar angle too much far from the resonance value, since the accuracy on the phase term is related to the signal-tonoise ratio of the intensity signal. Outside resonance in fact, as Fig. 4a shows, a decrease of the amplitude f1, besides an increase of the bias term f0, cause the increase in the SNR to affect the error on the phase term a0. The average refractive index sensitivity is around Sn,a = 235°/RIU with a resolution rn,a around 2  105 RIU. The same polarization-modulation analysis was performed in air in the assembled prototype for a detection test of biotin/avidin binding reaction. In this setup the grating is rotated and kept fixed at resonance position. A rotating polarizer between source and sample-holder allows controlling the incident polarization. In our system, after the grating was fixed on the rotation stage by the custom holder, we chose an incident angle h of 40° and fixed the incident polarization a around 130°, close to the optimal value aopt for the selected incidence configuration. Afterwards measurements of the output current at different azimuth angles were performed in order to identify the resonance azimuth angle ures at which the plasmonic dip occurs. The resonance azimuth angle position was identified with a weighted centroid algorithm [24]. For the fixed polar angle h = 40°, the reflectivity minimum is located at ures = 50.4°. Therefore at the fixed resonance condition, the trend of the output current as a function of the incident polarization was collected and well fitted by the harmonic curve in Eq. (3), as Fig. 5 shows. The polarization angle was varied continuously in the range 0–180° collecting 2060 points in a time of about 2.5 min (estimated rate  11.4 pts/s) and the scan was performed before and after the flowing of the avidin solution in order to test the detection of the protein as a polarization shift of the output sinusoid. Output data as a function of polarization were fitted with a least square algorithm using Eq. (3) in order to estimate the phase shift Da0 with the corresponding error ra. The binding of the avidin to the biotin-end of the m-PEO-Cys biorecognizer layer causes a phase shift Da0,Avi = 2.1982° ± 0.0004°. By modeling the effective refractive index change Dneff with an effective medium approximation method [25], it is possible to estimate the corresponding phase sensitivity Sn,a = Da0/Dn and calculate the refractive index resolution rn,a = ra/Sn,a. The antifouling b-PEO-Cys layer is assumed to form a packed monolayer with refractive index nPEO = 1.47 [26]. In order to get an estimation of the layer thickness, a flat gold sample was grafted following the same procedure and analyzed with spectroscopic ellipsometry. Collected data were fitted by means of the ellipsometer software

Fig. 5. Output intensity for a polarization scan in the range 0–180°, 2060 points, on the bare grating coated with biotin-PEO-Cysteine (b-PEO-Cys) biorecognizer layer (blue line) and after avidin detection (red line). Fixed polar and azimuthal angles: h = 40°, u = 50.4°. In the inset graph: details of the phase shift. (For interpretation of color in Fig. 5, the reader is referred to the web version of this article.)

package WVASE32 (Woollam) considering a Cauchy model for the b-PEO-Cys monolayer in the optical range with Urbach tail adsorption. The resulting thickness is dPEO = 2.21 ± 0.04 nm. The same analysis was performed after exposition to avidin solution for the proper time. The adsorbed analyte is supposed to form a thin monolayer with refractive index nAvi = 1.45 [17] and the thickness from ellipsometric analysis is around dAvi = 2.36 ± 0.04 nm, for a total thickness of the coating layer dPEO+Avi = 4.57 ± 0.08 nm. The consequent effective index variation after avidin binding, calculated with an EMA approach, is Dneff,Avi = 79.2  104 RIU. Thus the sensitivity to functionalization of the whole sensing platform is given by Sn,a = Da0,Avi/Dneff,Avi = 277.6°/RIU, with a refractive index resolution around rn,a = 1  106 RIU. 5. Conclusions A novel grating-coupled SPR technique based on polarization interrogation has been tested for the detection of refractive index variations due to saline concentration or surface functionalization. A microfluidic cell has been fabricated by soft-lithography techniques and embodied onto gold gratings fabricated by interferential lithography and replicated onto resin substrates before metal evaporation. At first, measurements were performed in water environment with water solutions flowing through the cell during data acquisition on the optical bench of a spectroscopic ellipsometer. Afterwards this polarization-modulation technique was tested on an assembled prototype with a model biorecognition assay using the avidin/biotin reaction. A polarization scan was performed in air on the b-PEO-Cys functionalized sample before and after the exposition to avidin solution and the shift of the phase term was considered in order to detect the binding of the selected analyte. Since the incidence configuration is kept fixed during SPR analysis and the only degree of freedom is represented by polarization, this setup provides a sensing architecture which is much more amenable to miniaturization than other techniques based for instance on angular or wavelength interrogation. In addition, competitive performances in refractive index resolution and sensitivity are provided. By estimating the refractive index variation with an effective medium approach, it was possible to estimate the refractive index resolution of the SPR system around 1  106 RIU. Common values in literature for grating-coupled SPR devices [27] are around 106–107 RIU, thus the preliminary

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experimental results confirms the cutting-edge performance of the presented system. However this value strictly depends on the phase-shift resolution, which in turn is related to the noise of the detector and of the electronic chain connected for data transduction. The estimated resolution could be further improved up to at least one order of magnitude with the use of a low-noise photodetector with a greater signal-to-noise ratio or by further increasing data statistics with a shorter angular step of the polarization scan. In addition, as experimentally demonstrated in a previous work [14], the configuration of merging double-SPP excitation could provide a further improvement in sensitivity up to one order of magnitude with respect to single SPP excitation. Therefore a refractive index resolution down to 107 RIU is estimated to be easily accessible. Acknowledgement This work has been supported by a Grant from the strategic project of University of Padova PLATFORMS (PLAsmonicnano-Textured materials and architectures FOR enhanced Molecular Sensing’’). References [1] R. Daw, J. Finkelstein, Nature 442 (2006) 367. [2] H. Raether, Surface Plasmons on Smooth and Rough Surfaces and on Gratings, Springer-Verlag, Berlin, Heidelberg, New York, 1988. [3] S.A. Maier (Ed.), Plasmonics - Fundamentals and Applications, Springer-Verlag, Berlin, Heidelberg, New York, 2007. [4] J. Homola, S.S. Yee, G. Gauglitz, Sens. Actuators B 54 (1999) 3–15. [5] E. Kretschmann, Z. Phys. 241 (1971) 313–324. [6] J. Homola, Surface Plasmon Resonance Based Sensors, Springer-Verlag, Berlin, Heidelberg, New York, 2006.

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